Hello Tyson, welcome to Armaholic! I think you might have a classic locality issue.

If you look at the top of both commands's wiki pages (playSound, createMarker) you can see an icon with the letter E in both, which indicates the command's effect. For createMarker it has a G that stands for global, and for playSound it has a L that stands for local.

Basically explained, commands with global effect will be broadcasted across all clients (aka globally), but local commands will stay on the computer where they were executed. Since you're hosting the game, only you get the effect of the local command. You can solve it by replacing your playSound line with this:

"Alarm" remoteExec ["playSound"];

This whole concept is called locality, and you could search "Arma 3 Locality" to find out more, or you can ask here if you'd like to find out more. If you don't want to deal with all the learning, just familiarize yourself with the remoteExec command and you should be able to solve most of these problems by yourself.

Sometimes I like to think as I started the whole "earplugs" thing.

W0lle: The only advice I can give you is: Do not try to understand BI. You will not succeed and it only makes your brain go boom. I would even go so far and say that not even they understand their own actions :-D.